Grinding of solid materials such as the raw materials for cement from crushed rock are well known and include ball mills, roller mills, pinch mills, hammer mills, jet mills, cone mills, etc. Examples of jet mills are British patent publication 2,232,096A published Dec. 5, 1990 and U.S. Pat. No. 3,877,647. Certain of the several types of mills have been combined in the past in various ways to achieve certain advantages of efficiency, product improvement, output, apparatus longevity, and the like.